docs: add tips and use cases section to llms.txt page

[devin-ai-integration]

Summary

Adds a new "Tips and use cases" section to the llms.txt documentation page with two practical examples:

1. Link to SDK documentation for LLMs - Shows how to add a Button component that links to a filtered llms-full.txt for users who want to feed SDK docs to AI tools instead of raw HTTP requests

2. Add a dropdown for llms-full.txt - Shows how to configure a navbar dropdown with links to different language-filtered versions of llms-full.txt

This addresses the feedback from the Slack thread about helping customers understand practical applications of the llms.txt filtering capabilities.

Review & Testing Checklist for Human

• Verify the Button component example syntax is correct (matches existing Button component docs)
• Verify the navbar-links dropdown YAML syntax is correct (matches site-level-settings docs)
• Check the preview to ensure the new section renders properly with code blocks and syntax highlighting

Test plan: Navigate to the llms-txt page in the preview and verify both code examples display correctly with proper formatting.

Notes

The llms-ignore/llms-only content was already covered in the existing "Control visibility" section, so that wasn't added.

Link to Devin run: https://app.devin.ai/sessions/668f937abb7f43d3b29b753b115eab96
Requested by: Fern Support

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-8f7ee260-cf1f-4f61-b9e1-99b520af7e59.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/llms-txt

[github-actions]

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